Pôle Alpin Risques Naturels (PARN) Alpes–Climat–Risques Avec le soutien de la Région Rhône-Alpes (2007-2014)
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Réf. Rabatel & al. 2013 - A

Référence bibliographique
RABATEL A., LETREGUILLY A., DEDIEU JP., ECKERT N. (2013). Changes in glacier equilibrium-line altitude in the western alps from 1984 to 2010: evaluation by remote sensing and modeling of the morpho-topographic and climate controls. The Cryosphere, vol. 7, pp. 1455-1471. PDF

Abstract: We present time series of equilibrium-line altitude (ELA) measured from the end-of-summer snow line altitude computed using satellite images, for 43 glaciers in the western Alps over the 1984–2010 period. More than 120 satellite images acquired by Landsat, SPOT and ASTER were used. In parallel, changes in climate variables, summer cumulative positive degree days (CPDD) and winter precipitation, were analyzed over the same time period using 22 weather stations located inside and around the study area. Assuming a continuous linear trend over the study period: (1) the average ELA of the 43 glaciers increased by about 170 m; (2) summer CPDD increased by about 150 PDD at 3000ma.s.l.; and (3) winter precipitation remained rather stationary. Summer CPDD showed homogeneous spatial and temporal variability; winter precipitation showed homogeneous temporal variability, but some stations showed a slightly different spatial pattern. Regarding ELAs, temporal variability between the 43 glaciers was also homogeneous, but spatially, glaciers in the southern part of the study area differed from glaciers in the northern part, mainly due to a different precipitation pattern. A sensitivity analysis of the ELAs to climate and morpho-topographic variables (elevation, aspect, latitude) highlighted the following: (1) the average ELA over the study period of each glacier is strongly controlled by morphotopographic variables; and (2) the interannual variability of the ELA is strongly controlled by climate variables, with the observed increasing trend mainly driven by increasing temperatures, even if significant nonlinear, low-frequency fluctuations appear to be driven by winter precipitation anomalies. Finally, we used an expansion of Lliboutry’s approach to reconstruct fluctuations in the ELA of any glacier of the study area with respect to morpho-topographic and climate variables, by quantifying their respective weight and the related uncertainties in a consistent manner within a hierarchical Bayesian framework. This method was tested and validated using the ELA measured on the satellite images.

Mots-clés

Organismes / Contact

Authors / Auteurs :

  • Antoine Rabatel, Univ. Grenoble Alpes, CNRS, LGGE, UMR5183, 38000 Grenoble, France
  • Anne Letréguilly, Univ. Grenoble Alpes, CNRS, LGGE, UMR5183, 38000 Grenoble, France
  • Jean-Pierre Dedieu, IRD, LTHE, UMR5564, 38000 Grenoble, France
  • Nicolas Eckert, IRSTEA, UR ETGR, 38000 Grenoble, France

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
  glacier / evolution of ELA / Mass Balance    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
France Alpes 43 glaciers     1984-2010

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations

 

Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)



(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations

 Evolution of CPDD (Cumulative Positive Degree-Days) :

Summer CPDD present an increasing trend, averaging 5.3±1.9 CPDDyr−1 at 3000ma.s.l. assuming a linear trend over the period 1984–2010. Such an increase (150±1.9) in the CPDD over the study period is the equivalent of an additional energy supply of 14±6Wm−2 , which can result in an additional ablation of about 0.5mw.e.

Evolution of winter precipitation :

Regarding winter precipitation, no significant trend was observed over the study period. Over the study period, maximum winter precipitation occurred in 2001, and the same value was almost reached in 1995. These two years correspond to low ELAs, but not to the lowest of the whole time series, which occurred in 1984 and 1985, and appears to be associated more with cooler summer temperatures.

Evolution of the ELA (Equilibrium Line-Altitude) :

Considering all the glaciers, the average ELA for the whole period was located at 3035±120ma.s.l., i.e. the interannual variability of the average ELA of all glaciers was high (120m on average). The difference between extreme years was as high as 460 m, with the lowest average ELA measured in 1984 (2790±180ma.s.l.), and the highest average ELA measured in 2003 (3250±135ma.s.l.). In addition, over the study period, the ELA time series showed an average increasing trend of 6.4myr−1.

Conclusion :

Assuming a linear trend over the study period: the average ELA increased by about 170 m; the summer CPDD increased by about 150 PDD at 3000ma.s.l., while winter precipitation remained rather stationary.

Evolution du cumul des jours d’été à température positive :

On observe une tendance d’accroissement dans l’évolution du cumul des jours d’été à température positive. Cet accroissement se traduit par une augmentation moyenne de 5.3±1.9 CPDD par an à 3000m (en prenant en compte une évolution linéaire) sur la période étudiée : 1984-2010. Une telle augmentation peut être comparée à un apport énergétique supplémentaire de 14±6W par m², ce qui se traduit par une augmentation de l’ablation de l’ordre de 0.5me.e

Evolution des précipitations hivernales :

En observant les précipitations hivernales, aucune tendance significative n’a été décelée sur la période étudiée. Ainsi, les précipitations hivernales les plus importantes sont répertoriées en 2001, et des précipitations d’une ampleur presque identique furent enregistrées en 1995. Ces deux années correspondent à des LEA basses, mais qui ne furent pas les plus basses de la période (enregistrées en 1984 et 1985) qui, quant à elles, sont associées à des températures estivales plus fraîches.

Evolution de la LEA (ligne d’équilibre glaciaire) :

En étudiant l’ensemble des glaciers, la LEA moyenne pour la période étudiée se trouvait à 3035±120m. La variabilité interannuelle de la LEA moyenne était donc importante (120m en moyenne). La différence entre les années les plus extrêmes était de 460 m au maximum, avec la LEA moyenne la plus basse mesurée en 1984 (2790±180m), et la plus haute en 2003 (3250±135m). Sur la période étudiée, on observe une augmentation moyenne de la hauteur de la LEA de 6.4m par an.

Conclusion :

En supposant que la tendance d’évolution de la LEA sur notre période soit linéaire, on observe une augmentation moyenne de la position de la LEA de 170m ; le CPDD d’été augmente d’environ 150 jours à 3000m alors que les précipitations hivernales restent stables.

Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 A sensitivity analysis of the ELAs to climate and morpho-topographic variables (elevation, aspect, latitude) highlighted the following: (1) the average ELA over the study period of each glacier is strongly controlled by morphotopographic variables; and (2) the interannual variability of the ELA is strongly controlled by climate variables, with the observed increasing trend mainly driven by increasing temperatures, even if significant nonlinear, low-frequency fluctuations appear to be driven by winter precipitation anomalies.
Une analyse de la sensibilité de la LEA aux variables climatiques et morpho-topographiques (altitude, caractéristiques, latitude) mis en avant les conclusions suivantes : (1) la LEA moyenne de chaque glacier sur la période d’étude est fortement dépendante des variables morpho-topographiques ; (2) la variabilité interannuelle de la LEA est fortement dépendante des variables climatiques, et en particulier de l’augmentation des températures, même si, bien que suivant une tendance non linéaire, les fluctuations à basse fréquence sont, quant à elle, fortement dépendantes des anomalies dans les précipitations hivernales.

In the current study, 43 glaciers located in the French Alps or just next to the border with Switzerland and Italy were selected (Table 1, Fig. 1). The selection was based on the following criteria: (1) glaciers had to have a high enough maximum elevation to allow observation of the snow line every year during the study period; (2) glaciers with all aspects had to be represented; and (3) glaciers located in all the glacierized mountain ranges in the French Alps from the southernmost (44°500 N) to the northern-most (46°000 N) had to be included. (...)

A total of 122 images of the 43 glaciers were used to cover the 27 yr study period. Unfortunately, in some years usable images were not available for all the glaciers because of (1) cloud cover hiding the underlying terrain; and (2) snowfalls that can occur in late summer and completely cover the glaciers. The images we used were acquired by the following satellites: Landsat 4TM, 5TM, 7ETM+, SPOT 1 to 5 and ASTER, with spatial resolutions ranging from 2.5 to 30m.

Dans cette étude, 43 glaciers, situés dans les Alpes Françaises ou à la frontière avec la Suisse et l’Italie, furent sélectionnés. La sélection fut basée sur les critères suivant : (1) les glaciers devaient avoir une altitude maximum assez élevée afin de permettre l’observation de la ligne de neige chaque année durant la période d’étude ; (2) chaque type de glaciers devait être représenté ; (3) des glaciers de chaque zone glaciaire dans les alpes françaises, du sud (44°500N) jusqu’au nord (46°000N) devaient être inclus à l’étude. Un total de 122 images des 43 glaciers fut utilisé pour couvrir les 27 ans de la période d’étude. Malheureusement, quelques images de glaciers sont manquantes pour certaines années. Ce manque de données s’explique de 2 façons : (1) La couverture nuageuse empêche de voir le terrain d’étude. (2) Les chutes de neige qui peuvent survenir en fin d’été peuvent recouvrir complètement les glaciers, empêchant par la même l’identification de la ligne de neige. Les images utilisées furent enregistrées par les satellites suivants : Landsat 4TM, 5TM, 7ETM+ , SPOT1 à 5 et ASTER, avec des résolutions spatiales comprises entre 2.5 et 30m.

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
 
 

(4) - Remarques générales
 

(5) - Syntèses et préconisations
 

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